The present invention relates to semiconductor processing and integrated circuits. More particularly, the present invention relates to semiconductor circuits that are susceptible to ionizing radiation.
Older semiconductor processing technologies produced integrated circuits that were highly susceptible to damage from ionizing radiation. Such ionizing radiation is emitted from a multitude of galactic sources (e.g., the Sun, etc.) and exists above the ionosphere, and is also emitted when nuclear weapons are detonated. The important consequence of such radiation susceptibility is that these integrated circuits were not well suited for use in satellites or in military applications. They could, therefore, be freely sold and exported without the fear that they would be used militarily against the United States or its allies.
In contrast, some state-of-the-art semiconductor processing technologies currently produce integrated circuits that are highly tolerant to damage from ionizing radiation. Such tolerance results, among any other reasons, from a decrease in semiconductor-feature size (e.g., interconnect line width, etc.) in integrated circuits. In particular, the xe2x80x9cgate oxidexe2x80x9d or xe2x80x9cgate insulatorxe2x80x9d in field effect transistors (FETs) has thinned to the point where, by virtue of such thinness, it is inherently tolerant to ionizing radiation.
The relatively high radiation tolerance of state-of-the-art circuits is no benefit to most users and for most applications. This characteristic does, however, allow such circuits to be used in aerospace and military applications. In fact, such circuits may be so radiation tolerant that Department of Defense export restrictions (ITAR) are implicated so that such circuits can not be freely sold and exported. To the extent that a commercial CMOS fabricator is restricted from freely exporting its chips, it suffers financially.
Thus, a need exists for semiconductor circuits that exhibit increased susceptibility to ionizing radiation, but that retain the advantages that accrue from contemporary processing methods.
Some embodiments of the present invention provide semiconductor circuits that have increased susceptibility to ionizing radiation without the costs and disadvantages of techniques in the prior art. In particular, the illustrative embodiment of the present invention is an integrated circuit that possesses the benefits of contemporary processing technologies (e.g., small feature size, etc.) yet is advantageously irreparably damaged by ionizing radiation. Thus, some integrated circuits made in accordance with the present teachings can pass Department of Defense export restrictions and can be freely sold and exported.
In accordance with an illustrative embodiment of the present invention, a radiation-susceptible integrated circuit comprises a radiation sensor, a differential amplifier and a circuit disabler that are operable, collectively, to disable an IC when it is exposed to a sufficient amount of ionizing radiation. The radiation sensor is operable to generate a first signal and a second signal, and the differential amplifier is operable to receive the first and second signals and to generate a third signal having a value that is a function of the first and second signals. The circuit disabler, which is electrically connected to the differential amplifier, is operable to receive the third signal from the differential amplifier and to disable the integrated circuit, or not, responsive to the value of the third signal.
In the illustrative embodiment, the radiation sensor comprises two devices that have a different tolerance to ionizing radiation. In one embodiment, such devices are transistors that have different structures such that they exhibit a different susceptibility to ionizing radiation. When exposed to a total dose of ionizing radiation that exceeds the radiation tolerance of one of the transistors but not the other, only the more radiation-susceptible transistor will exhibit an increase in leakage current.
The differential amplifier receives, as its input, two signals from the radiation sensor. In the illustrative embodiment, those two signals are the output (i.e., the leakage current) from the two devices (e.g., transistors). The differential amplifier is operable to generate an output signal having a value that is indicative of a difference or offset that exists between the input signals. Such an offset results when the IC is exposed to an amount of ionizing radiation sufficient to cause only one of the devices in the radiation sensor to exhibit an increase in leakage current. In one embodiment, the differential amplifier generates a xe2x80x9chighxe2x80x9d-voltage signal when an offset (increased leakage current from one device) is observed, and a xe2x80x9clowxe2x80x9d-voltage signal in the absence of an offset.
The output signal from the differential amplifier is received by a circuit disabler. The circuit disabler is activated, or not, as a function of the value of the output signal. For example, a high-voltage output signal from the differential amplifier activates the circuit disabler, while a low-voltage output signal does not. When activated, the circuit disabler is operable to cause the integrated circuit to stop functioning, and preferably causes irreparable damage thereto.
The circuit disabler can be implemented in a variety of ways. For example, in a first embodiment, the circuit disabler comprises a switching FET that shorts power to ground in the IC when activated. In a second embodiment, the circuit disabler comprises a switching FET that shorts signal to ground in the IC when activated. In a third embodiment, the circuit disabler comprises circuitry for disabling the IC""s clock driver, and in a fourth embodiment, the circuit disabler comprises circuitry suitable for activating the chip-reset function.
A further illustrative embodiment of the present invention is a method that comprises comparing a first signal indicative of a first current leakage with a second signal indicative of a second current leakage; and generating a third signal having a value that is a function of the difference between the first and second signals. In particular, the third signal has a first value when the difference between the first and second signals meets or exceeds a threshold difference, and has a second value when the difference between the first and second signal is less than the threshold difference. The first and second current leakages are indicative of an amount of radiation exposure. In a further embodiment of the method, an integrated circuit is disabled when the third signal has the first value and is not disabled when the third signal has the second value.